None
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The present invention relates generally to a method for filtration of polyol dispersions and, more particularly, to an index filtering method for filtration of graft polyol dispersions. Graft polyols are generally defined as vinyl polymer dispersions in polyether polyols and are also known as polymer polyols, as disclosed generally in U.S. Pat. Reissue No. Re. 33,291. Formation of graft polyols generally comprises the in situ polymerization of a polyether polyol having induced unsaturation, commonly known as a macromer, and an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers in the presence of a carrier polyol, a reaction moderator, and a free radical polymerization initiator. Microscopic examination of graft polyol dispersions reveals that they include numerous desirable small particles, these desirable particles are typically in a size range of from 0.1 to 2.5 microns in diameter. Graft polyol dispersions, however, typically also contain particles that may range from 0.04 to 100.0 microns in diameter. The very large particles are not desirable. Graft polyols find great use in polyurethane foaming systems because of the advantageous properties they provide to foams. Many of these desirable characteristics are attributable to the desirable sized particles found in graft polyol dispersions.
One disadvantage with graft polyol dispersions is that during the process of manufacturing them large particles and large aggregates of particles are often formed, both of which are believed to cause great difficulties in typical polyurethane foaming machines utilized to prepare foam from these graft polyols. The problem becomes particularly important when one utilizes carbon dioxide as a blowing agent for the polyurethane foam. The foaming heads of carbon dioxide foam systems have much tighter tolerances than those of typical water-blown, acetone-blown, or chlorinated fluorocarbon-blown foam systems. The carbon dioxide-blown foam machines are particularly useful because of recent concerns about environmental damage from chlorinated fluorocarbon blowing agents, which resulted in environmental regulations that now ban them in the United States.
To alleviate the difficulties that can be caused in a foaming system by the large particles and the large aggregates of graft polyol particles, it is common to pre-filter graft polyols prior to their utilization in a foaming system. Practical considerations require that the graft polyols undergo at least several filtration steps in the typical procedure prior to utilization in a foaming system. Often, the graft polyols are filtered as they are transferred from their storage tanks to tanker trucks or railcars for distribution to foam manufacturers. A second filtration is often carried out as the graft polyol dispersion is unloaded from the railcar or the tanker truck into the foam manufacturer""s facility. A third filtration is often carried out as the graft polyol is entering the foam system. For a graft polyol to perform well in most carbon dioxide-blown foam systems it is necessary that the graft polyol pass through the pre-filter of the carbon dioxide foam machine for an extended period of time, typically in excess of 4 hours, without plugging the filter. These filters usually have hole sizes of about 100 microns for Novaflex, manufactured by Hennecke-Bayer, and Beamech machines, and 150 microns for Cardio, manufactured by Cannon-Viking, machines. Typically the pre-filter must be switched when the pressure drop across this pre-filter reaches about 70 psig and manufacturers of foams would like to change these pre-filters as infrequently as possible.
Filtration of graft polyols presents a number of difficulties in part due to the characteristics of graft polyol particles. First, it is necessary to remove only the oversized particles from the graft polyol dispersion, but not the desirably smaller sized particles discussed above because they provide the beneficial characteristics. Most preferably, the filtered graft polyol dispersion will exclude mainly particles having a size of greater than 25 microns with minimal removal of smaller particles. Second, by its nature the carrier polyol in the dispersion is viscous and the presence of the graft polyol particles makes the graft polyol dispersion much more viscous. Graft polyol dispersions are also very sticky. Third, the graft polyol particles tend to be deformable at the filtration temperatures that are used to reduce the viscosity such that they can deform under pressure which leads them to either rapidly plug a typical filter media or to pass through a typical filter media with a defined pore size even though the size of the particle is larger than the pore size. In the present invention it has been found that the use of depth filtration filter media which has a maximum average mean flow pore size in the range of about 15 to 75 microns is preferable with the most preferable being a mean flow pore size ranging from 15 to 50 microns to minimize the plugging in the pre-filters of carbon dioxide blown machines, which as noted above generally have hole sizes in the range of 100 to 150 microns, and is sufficient to insure continuous operation at the foam head. The mean flow pore size as used in the present specification and claims is defined as the diameter of the smallest pore needed free to have half the overall area of the filter sample free. The test for this as defined in ASTM Method F-316. The phenomena that a material filtered through a nominal 25 micron pore size can in fact plug a filter media having much larger holes of from 100 to 150 microns is believed to be attributed to the tendency to bridge or form agglomerates across the holes of the coarser media, which is promoted by large particles, large aggregates of particles, and particle stickiness. Bridging phenomena resulting in plugged filtration media is a well-known phenomenon documented in many filtration references. Furthermore the plugging tendency in the coarser screen can also be promoted by particles and particle aggregates that are larger than the filtration media pores deforming and passing through the filtration media.
In the past, filtration methods for graft polyols have included in-line screen filters, bag filters, and cartridge filters. For the reasons discussed above, however, all of these methods suffer from deficiencies that make them largely impractical for graft polyol dispersions in general. They tend to plug rather rapidly with graft polyol, it is difficult and time consuming to stop the filtration system, remove the plugged filters or unplug the filters or replace them, and then reassemble the filtration system. Typically, to avoid plugging of such filters, they are staged, with the initial stage taking out the coarsest fraction, then a second finer stage removing another finer cut of material. This reduces the frequency of changing of the filter media overall, but still results in a large area requirement. In some cases self-cleaning filters could potentially be used to filter graft polyols. These self cleaning filters, such as from Ronningen-Petter, Inc., have a wiper which cleans the surface of a cylindrical filtration screen continuously to avoid blockage, and the filter periodically purges out the build-up of trapped material. In general, these devices do not use fine enough screens to produce product suitable for carbon dioxide-blown foam machines. When they do have fine screens they suffer from low throughputs, high pressure drops that force particles to deform and pass through the media, and the need to frequently clean the screens which often plug. For many graft polyol dispersions they are effectively impractical.
Because of the difficulties described above it would be advantageous to develop a method for filtration of graft polyols that allowed for rapid filtration of the graft polyol to the appropriate size, significant throughput in the system, ease of filter media replacement, and long-term stability of the filtered graft polyol dispersion.
In a first embodiment, the present invention is a method for index filtration of a graft polyol comprising the steps of: providing an index filtration system having a first reservoir and a second reservoir; securing a first portion of a depth filtration filter media between the first and second reservoirs and forming a liquid tight seal between the first reservoir and the filter media; introducing a graft polyol dispersion into the first reservoir; receiving the graft polyol dispersion in the second reservoir after it passes from the first reservoir and through the filter media; and moving the used first portion of the depth filtration filter media from between the first and second reservoirs and positioning a second clean portion of the depth filtration filter media between the first and second reservoirs.
In a second embodiment the present invention is a method for index filtration of a graft polyol comprising the steps of: providing an index filtration system having a first reservoir and a second reservoir; securing a first portion of a depth filtration filter media having a mean flow pore size of from 15 to 75 microns between the first and second reservoirs and forming a liquid tight seal between the first reservoir and the filter media; introducing a graft polyol dispersion into the first reservoir; passing the graft polyol dispersion through the filter media and receiving the graft polyol dispersion in the second reservoir after it passes from the first reservoir and through the filter media; and moving the first portion of the depth filtration filter media from between the first and second reservoirs and positioning a second portion of the depth filtration filter media between the first and second reservoirs. Because the present invention utilizes a depth filtration type of filtration media it also removes some particles that are smaller than the smallest pore size via the depth filtration mechanism.